Unraveling the rheology of inverse vulcanized polymers

Multiple relaxation times are used to capture the numerous stress relaxation modes found in bulk polymer melts. Herein, inverse vulcanization is used to synthesize high sulfur content (≥50 wt%) polymers that only need a single relaxation time to describe their stress relaxation. The S-S bonds in these organopolysulfides undergo dissociative bond exchange when exposed to elevated temperatures, making the bond exchange dominate the stress relaxation. Through the introduction of a dimeric norbornadiene crosslinker that improves thermomechanical properties, we show that it is possible for the Maxwell model of viscoelasticity to describe both dissociative covalent adaptable networks and living polymers, which is one of the few experimental realizations of a Maxwellian material. Rheological master curves utilizing time-temperature superposition were constructed using relaxation times as nonarbitrary horizontal shift factors. Despite advances in inverse vulcanization, this is the first complete characterization of the rheological properties of this class of unique polymeric material.

Copolymerization Procedure for Sulfur and Styrene -poly(S50-r-Sty50): To a 20 mL scintillation vial, elemental sulfur (2.5 g, 9.7 mmol) was added with a magnetic stir bar and heated to 135 °C in a thermostated oil bath until a clear yellow molten phase was formed.Uninhibited/purified styrene (2.5 g, 23.9 mmol) was added to the molten sulfur dropwise to prevent a sudden temperature drop.The polymerization proceeded for 6 hours at 130 °C until complete consumption of styrene was confirmed by 1 H NMR spectroscopy.Next, the reaction mixture was cooled to room temperature affording a viscous, red fluid.After the mixture was diluted in 10 mL of anhydrous THF, the solution was placed in a refrigerator overnight that induced precipitation of unreacted elemental sulfur which was removed by filtration.All volatiles were removed under reduced pressure and the polymer was dried under high vacuum overnight.(yield = 4.73 g, Mn = 1,100 g/mol, Đ = 1.24).
General Terpolymerization Procedure for Sulfur, DIB, and Sty -poly(S50-r-DIB10-r-Sty40): To a 20 mL scintillation vial, elemental sulfur (2.5 g, 9.69 mmol) was added with a magnetic stir bar and heated to 135 °C in a thermostated oil bath until a clear yellow molten phase was formed.1,3-Diisopropenylbenzene (DIB, 0.5 g, 3.16 mmol) was then injected into the molten phase via a syringe.Uninhibited/purified styrene (2 g, 19.1 mmol) was added next to the molten sulfur dropwise to prevent a sudden temperature drop.The polymerization proceeded until the viscosity prohibited continued mixing, after which the material cured at temperature for an additional hour.The reaction was cooled to room temperature before being cooled with liquid nitrogen to remove it from the vial for a quantitative yield.
General Copolymerization Procedure for Sulfur and DIB -poly(S50-r-DIB50): Elemental sulfur (2.5g, 9.69 mmol) was added to a 20 mL glass vial equipped with a magnetic stir bar and was heated to 165 °C in an oil bath until a clear yellow molten phase was formed.1,3-Diisopropenylbenzene (DIB, 2.5 g, 15.8 mmol) was then injected into the molten phase via a syringe.The resulting mixture was stirred at 165 °C for 8-10 minutes until stirring stopped due to the increased viscosity of the reaction mixture.After cooling to room temperature, the polysulfide was extracted from the vial to yield 4.89 g (97.8 %) of poly(S50-r-DIB50).
General Copolymerization Procedure for Sulfur and NBD2 -poly(S50-r-NBD250): Elemental sulfur (2.5 g, 9.69 mmol) was added to a 20 mL glass vial equipped with a magnetic stir bar and was heated to 165 °C in an oil bath until a clear yellow molten phase was formed.Norbornadiene (NBD2, 2.5 g, 13.56 mmol) was then added to the molten phase.The resulting mixture was stirred at 165 °C for 15 minutes until stirring stopped due to the increased viscosity of the reaction mixture.After cooling to room temperature, the polysulfide was extracted from the vial to yield 4.91 g (98.2 %) of poly(S50-r-NBD250).
General Terpolymerization Procedure for Sulfur, DIB, and NBD2 -poly(S50-r-DIB10-r-NBD240): Elemental sulfur (2.5 g, 9.69 mmol) was added to a 20mL glass vial equipped with a magnetic stir bar and was heated to 165 °C in an oil bath until a clear yellow molten phase was formed.Norbornadiene (NBD2, 0.5 g, 2.71 mmol) was then added to the molten phase.1,3-Diisopropenylbenzene (DIB, 2 g, 12.64 mmol) was then injected into the molten phase via a syringe.The resulting mixture was stirred at 165 °C for 15 minutes until stirring stopped due to the increased viscosity of the reaction mixture.After cooling to room temperature, the polysulfide was extracted from the vial to yield 4.91 g (98.2 %) of poly(S50-r-DIB10-r-NBD240).
Supplementary Figure 4: ssNMR results. 13C NMR shifts predicted for the partially reacted NBD2 monomer in poly(S-r-NBD2) using ACD 2018.The minor signal near 137 ppm observed in 13 C spectrum of previous figure is due to a small amount of unreacted NBD2 olefinic group.Supplementary Figure 32: Rheological results for poly(S50-r-DIB50) using new material for each frequency sweep.It was hypothesized that deviations in the loss modulus at higher frequencies (Fig. 4c in main text) were caused by sulfur subliming from the material (since sulfur is soluble in the polymer and is difficult to separate) or from sulfur bloom leading to sublimation over extended periods of elevated temperature exposure (Supplementary Fig. S17 50 -r-DIB 50 ) poly(S 50 -r-DIB 40 -r-NBD2 ) poly(S 50 -r-DIB 25 -r-NBD2 ) poly(S 50 -r-DIB 10 -r-NBD2 ) poly(S 50 -r-NBD2 50 )Supplementary Figure5: TGA Results.Thermograms of poly(S-r-DIB) and poly(S-r-NBD2) copolymers as well as poly(S-r-DIB-r-NBD2) terpolymers all with 50 wt% sulfur Results.Thermograms of poly(S-r-DIB) and poly(S-r-NBD2) copolymers as well as the poly(S-r-DIB-r-NBD2) terpolymer all with 70 wt% sulfur Supplementary Figure7: TGA Results.Thermograms of poly(S-r-DIB) and poly(S-r-Sty) copolymers as well as poly(S-r-DIB-r-Sty) terpolymers all with 50 wt% sulfur Results.Thermograms of poly(S-r-DIB) and poly(S-r-NBD2) copolymers as well as poly(S-r-DIB-r-NBD2) terpolymers all with 50 wt% sulfur : DSC Results.Thermograms of poly(S-r-DIB) and poly(S-r-NBD2) copolymers as well as the poly(S-r-DIB-r-NBD2) terpolymer all with 70 wt% sulfur : DSC Results.Thermograms of poly(S-r-DIB) and poly(S-r-Sty) copolymers as well as poly(S-r-DIB-r-Sty) terpolymers all with 50 wt% sulfur Supplementary Figure12: DSC Results.Thermogram of elemental sulfur showing a Tm peak near 120 °C curve of poly(S50-r-DIB50) at Tref=100 °C.Master curve is constructed from standard TTS.The relaxation time at the crossover is  = [8.5610−3 /] −1 = 116.8sec, the terminal viscosity is ~2.19x10 7Pa-s, and the rubbery plateau modulus is 2.01x10 5 Pa (black point marked by star).: Rheological results for poly(S50-r-DIB50) a) Cole-Cole plot showing the semicircular shape indicating that the material is Maxwellian and dominated by a single relaxation mode.b) Determination of the Maxwell relaxation time at each temperature displaying a maximum in loss modulus as predicted by the Maxwell model.c) Dimensionless master curve utilizing Maxwell relaxation times for horizontal shifting and empirical shifting along the vertical axis.Solid black and blue lines show the Maxwell model.d) Resulting shift factors showing the importance of the vertical shift factor : Rheological results for poly(S50-r-DIB50) using a linear fit to calculate Maxwell relaxation times with a nonzero intercept.a) Cole-Cole plot showing the semicircular shape indicating that the material is Maxwellian and dominated by a single relaxation mode.b) Determination of the Maxwell relaxation time at each temperature displaying a maximum in loss modulus as predicted by the Maxwell model.c) Dimensionless master curve utilizing Maxwell relaxation times for horizontal shifting and empirical shifting along the vertical axis.Solid black and blue lines show the Maxwell model.d) Resulting shift factors showing the importance of the vertical shift factor

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). a) Cole-Cole plot showing the semicircular shape indicating that the material is Maxwellian and dominated by a single relaxation mode.b) Determination of the Maxwell relaxation time at each temperature displaying a maximum in loss modulus as predicted by the Maxwell model.c) Dimensionless master curve utilizing Maxwell relaxation times for horizontal shifting and empirical shifting along the vertical axis.Solid black and blue lines show the Maxwell model.d) Resulting shift factors showing the importance of the vertical shift factor : Rheological results for poly(S50-r-DIB40-r-NBD210).a) Cole-Cole plot showing the semicircular shape indicating that the material is Maxwellian and dominated by a single relaxation mode.b) Determination of the Maxwell relaxation time at each temperature displaying a maximum in loss modulus as predicted by the Maxwell model.c) Dimensionless master curve utilizing Maxwell relaxation times for horizontal shifting and empirical shifting along the vertical axis.Solid black and blue lines show the Maxwell model.d) Resulting shift factors showing the importance of the vertical shift factor : Rheological results for poly(S50-r-DIB25-r-NBD225).a) Cole-Cole plot showing the semicircular shape indicating that the material is Maxwellian and dominated by a single relaxation mode.b) Determination of the Maxwell relaxation time at each temperature displaying a maximum in loss modulus as predicted by the Maxwell model.c) Dimensionless master curve utilizing Maxwell relaxation times for horizontal shifting and empirical shifting along the vertical axis.Solid black and blue lines show the Maxwell model.d) Resulting shift factors showing the importance of the vertical shift factor